Resonance local phonon mode and electron spin-lattice relaxation of formate-type free radicals studied by electron spin echo in Cd(HCOO)(2)center dot 2H(2)O crystal

摘要

The results of X-band electron spin resonance (ESR) and electron spin echo (ESE) measurements for free radicals generated in Cd(HCOO)(2)center dot 2H(2)O single crystal are presented. From ESR spectra analysis the radicals were identified as CO2- after x-ray irradiation and as HOCO after gamma-ray irradiation. The room temperature g-factors are: g(parallel to) = 1.9969 and g(perpendicular to) = 2.0024 for CO2- and g(1) = 2.0087, g(2) = 2.0029 and g(3) = 1.9960 for HOCO. Axial g-tensor symmetry for CO2- is due to fast reorientation of the radical molecule around the g(parallel to)-axis. Assignment of HOCO is confirmed by hyperfine splitting (A(max) = 0.4 mT) from a single distant proton. Spin lattice relaxation rate was determined from ESE measurements in temperature range 4-250 K. Both radicals relax via local resonance mode lying within acoustic phonon branch. The existing theories of electron spin-lattice relaxation via local resonance mode are critically reviewed and compared with experimental data. A new approximation is proposed giving local mode energy (h) over bar omega(R) = 56 cm(-1) for CO2- and (h) over bar omega(R) = 44 cm(-1) for the HOCO-radical.